It is known in ellipsometry and polarimetry to impinge electromagnetic radiation onto a sample at an oblique angle, and collect electromagnetic radiation which reflects from the sample, then via detected change in the polarization state determine properties of the sample. Ellipsometer and Polarimeter Systems sequentially comprise a Source of a beam electromagnetic radiation, a Polarizer element, optionally a compensator element, a sample system, optionally a compensator element, an Analyzer element and a Detector System. It is noted that presence of at least one compensator is a distinguishing factor between ellipsometer and polarimeter systems.
The elements identified as a source of electromagnetic radiation, the polarizer and the sequentially first optional compensator can be considered to form, as a group, a Polarization State Generator (ie. PSG), and the sequentially second optional compensator, analyzer and detector can be considered, as a group, to form a Polarization State Detector (ie. PSD).
A problem which occurs in practicing ellipsometry or polarimetry, where a sample being investigated is not effectively infinitely thick, is that detected electromagnetic radiation which reflects from a sample includes components which reflect not only from the actual sample surface, and perhaps interfaces between thin films thereupon, but also from the back side of the substrate. Said reflection from the substrate backside confuses interpretation of the results, and while such can be accounted for in a mathematical model of the sample, it is often preferable to block said backside reflections and avoid the confusing effects thereof.
One approach to preventing backside reflections is to physically roughen the backside, however this approach alters the sample. The invention disclosed herein provides a simple approach to avoiding the effect of backside reflections without requiring sample modification.
With the present invention in mind a Search was conducted for patents that disclose means for blocking backside reflections from entering a detector. Patent Application No. 2002/0113200 A1 was identified as an aperture 103A is disclosed which can be placed near a detector to block entry of one of two beams from different sources. U.S. Pat. No. 3,799,679 to Simko is disclosed as an iris (38) is present near a detector which can be adjusted to block entry of backside reflection thereinto. Patents to Meeks, U.S. Pat. Nos. 6,130,749, 6,198,533 and 6,392,749 are disclosed for the presence of a hole 2022 in an integrating sphere near, but not atop a sample. U.S. Pat. No. 6,088,092 to Chen et al. is disclosed as it applies a spatial filter (28) to block backside reflection entry into a detector. U.S. Pat. No. 6,088,104 to Peterson is disclosed as a blocking element (B) is present which can be used to block electromagnetic radiation entry to a detector. U.S. Pat. No. 6,097,482 to Smith et al. is disclosed as it applies baffles to block light entry to a detector. U.S. Pat. No. 6,166,808 to Greve is disclosed as it describes use of an aperture near a detector to block backside reflections entry to a detector. A U.S. Pat. No. 3,857,637 to Obenredder, was identified by the Examiner in prosecution of the Parent application hereto, Ser. No, 10/731,202. It is noted that the opaque element (35) therein is shown in FIG. 3 thereof to be offset from contact with the top surface (29) of the sample glass (21). Further, in Col. 6, Lines 45-51 of the 637 patent it is stated that . . . an opaque member (35) such as a metal washer may be positioned adjacent the top surface (see FIGS. 2 and 3). In the instance where the washer is used, it would be advantageously positioned in the return tube (107) to shield the detector surface from the beam reflected from the bottom surface of the glass. However, said 637 patent does not identify or suggest applying such a mask in Ellipsometer and the like systems.
U.S. Pat. No. 5,298,974 to Chandley describes an apparatus for determining the surface topology of an article. FIG. 2 thereof suggests that a Mask-like element with a silt (5) therein can be placed into direct contact with a flat transparent article for the purpose of blocking reflections of an electromagnetic beam from the surface thereof other than those from the surface of the article. Said 974 patent does not identify or suggest applying such a mask in Ellipsometer and the like systems.
Another Published Application is WO 2005/088272A1 by Nanofilm Technologie. This publication is likely the best art found, and discloses blocking electromagnetic radiation backside reflections from a sample by a Mask which is offset from the surface of said sample, in an ellipsometer system. Also, U.S. Pat. No. 3,857,637 describes use of a mask offset from a sample in a non-ellipsometric setting and was cited by the Examiner in prosecution of the Parent application Ser. No. 10/731,202, which it is noted, in contrast, Claims a mask placed directly on the surface of a sample.
In addition, with the present invention in mind, it is further disclosed that Co-Pending application Ser. No. 11/288,785 from which this application is a CIP, is included herein by reference.
Even in view of the known prior art, need remains for a simple to practice method for avoiding effects of sample backside reflections which does not require sample, or investigating system alteration.